Implantable medical device having electromagnetic interference filter device to reduce pocket tissue heating

ABSTRACT

An active implantable medical device (AIMD) for use with a medical lead carrying at least one lead electrode. The AIMD comprises interior electronic circuitry configured for performing a medical function via the medical lead, an electrically conductive case containing the interior electronic circuitry, at least one electrical terminal configured for electrically coupling the electronic circuitry respectively to the lead electrode(s), and an inductive element coupled in series between the electrical terminal(s) and the case. The inductive element is configured for hindering the shunting of electrical current from the at least one electrical terminal to the case that has been induced by electromagnetic interference (EMI) impinging on the medical lead.

RELATED APPLICATION DATA

The present application claims the benefit under 35 U.S.C. §119 to U.S.provisional patent application Ser. No. 61/733,347, filed Dec. 4, 2012.The foregoing application is hereby incorporated by reference into thepresent application in its entirety.

FIELD OF THE INVENTION

The present invention relates to implantable medical devices, and inparticular, techniques for preventing heating of the tissue pocketsurrounding an implantable medical device in the presence of MagneticResonance Imaging.

BACKGROUND OF THE INVENTION

Active implantable medical devices (AIMDs) find applicability inneurostimulation systems that deliver electrical stimuli to body nervesand tissues for the therapy of various biological disorders, such aspacemakers to treat cardiac arrhythmia, defibrillators to treat cardiacfibrillation, cochlear stimulators to treat deafness, retinalstimulators to treat blindness, muscle stimulators to producecoordinated limb movement, spinal cord stimulators to treat chronicpain, cortical and deep brain stimulators to treat motor andpsychological disorders, and other neural stimulators to treat urinaryincontinence, sleep apnea, shoulder sublaxation, etc. AIMDs aretypically implanted within a tissue pocket of the patient, and connectedto neurostimulation leads that are implanted at a target stimulationsite remote from the tissue pocket.

AIMDs that are used in neurostimulation systems take the form of twogeneral types: fully implanted and radio-frequency (RF)-controlled. Thefully implanted AIMD contains the control circuitry, as well as a powersupply, e.g., a battery, all within an implantable pulse generator (IPG)connected to one or more leads with one or more electrodes forstimulating tissue, so that once programmed and turned on, the IPG canoperate independently of external hardware. The IPG is turned on and offand programmed to generate the desired stimulation pulses from anexternal programming device using transcutaneous electromagnetic links.In contrast, the RF-controlled AIMD includes an external transmitterinductively coupled via an electromagnetic link to an implantedreceiver-stimulator connected to one or more leads with one or moreelectrodes for stimulating tissue. The power source, e.g., a battery,for powering the implanted receiver, as well as control circuitry tocommand the receiver-stimulator, is contained in the externalcontroller—a hand-held sized device typically worn on the patient's beltor carried in a pocket. Data/power signals are transcutaneously coupledfrom a cable-connected transmission coil placed over the implantedreceiver-stimulator. The implanted receiver-stimulator receives thesignal and generates the stimulation.

AIMDs typically incorporate a sealing enclosure or case (commonlyreferred to as a “can”) that contacts tissue when implanted within thepatient. This enclosure is constructed from a biocompatible material,and typically a metallic material, such as titanium. The interiorcomponents contained within the case are typically electronic circuitsdesigned for processes, such as physiological signal sensing, diagnosis,data storage, therapy delivery, and telemetry. The case serves toisolate the interior components, which are typically not biocompatible,from the biological environment. In some AIMDs, the case also serves asa common or return electrode that allows sensing and delivery ofstimulation energy. This practice is commonly referred to as “monopolar”or “unipolar” sensing or therapy.

AIMDs are routinely implanted in patients who are in need of MagneticResonance Imaging (MRI). Thus, when designing implantableneurostimulation systems, consideration must be given to the possibilitythat the patient in which AIMD is implanted may be subjected to EMIgenerated by MRI scanners, which may potentially cause damage to theAIMD, as well as discomfort to the patient. In particular, in MRI,spatial encoding relies on successively applying magnetic fieldgradients. The magnetic field strength is a function of position andtime with the application of gradient fields throughout the imagingprocess. Gradient fields typically switch gradient coils (or magnets) ONand OFF thousands of times in the acquisition of a single image in thepresence of a large static magnetic field. Present-day MRI scanners canhave maximum gradient strengths of 100 mT/m and fast switching times(slew rates) of 150 mT/m/ms, which can result in induced voltages withfrequency content comparable to stimulation therapy frequencies. TypicalMRI scanners create gradient fields in the range of 100 Hz to 30 KHz,and Radio Frequency (RF) fields of 64 MHz for a 1.5 Tesla scanner and128 MHz for a 3 Tesla scanner.

To a certain extent, the AIMD case provides protection againstelectromagnetic interference (EMI) from environmental sources andmedical diagnostic tools, such as MRI scanners. However, the strength ofthe gradient magnetic field may be high enough to induce voltages (5-10Volts depending on the orientation of the lead inside the body withrespect to the MRI scanner) on to the stimulation lead(s), which inturn, are seen by the AIMD electronics. If these induced voltages arehigher than the voltage supply rails of the AIMD electronics, therecould exist paths within the AIMD that could induce current through theelectrodes on the stimulation lead(s), which in turn, could causeunwanted stimulation to the patient due to the similar frequency band,between the MRI-generated gradient field and intended stimulation energyfrequencies for therapy, as well as potentially damaging the electronicswithin the AIMD. To elaborate further, the gradient (magnetic) field mayinduce electrical energy within the wires of the stimulation lead(s),which may be conveyed into the circuitry of the AIMD and then out to theelectrodes of the stimulation leads via the passive charge recoveryswitches. For example, in a conventional neurostimulation system, aninduced voltage at the connector of the AIMD that is higher than AIMDbattery voltage (˜4-5V), may induce such unwanted stimulation currents.RF energy generated by the MRI scanner may induce electrical currents ofeven higher voltages within the AIMD.

In some embodiments, the induced RF electrical current is shunted to theAIMD case to protect AIMD internal components. However, the inducedelectrical current may be collected in an additive fashion, and resultin significant RF current flow from the AIMD case into the tissue pocketsurrounding the AIMD case. This, in turn, results in heating of thetissue, with the potential for patient discomfort or even tissue damage.For neurostimulation systems that use more than one lead electrodeconnection in the AIMD, the amount of impinging RF current from theleads may increase as the number of lead electrodes increases, providedthat the RF current arrives at the AIMD with similar phases from thevarious lead electrodes. This potentially exacerbates this heatingphenomenon for implanted neurostimulation systems with a large number oflead electrodes (e.g., multi-lead AIMDs having eight or more electrodesper lead).

There, thus, remains a need to prevent RF current from being conveyed tothe case electrically connected to the internal electronics containedwithin an AIMD.

SUMMARY OF THE INVENTION

In accordance with the present inventions, an active implantable medicaldevice (AIMD) for use with a medical lead carrying at least one leadelectrode is provided. The AIMD comprises interior electronic circuitryconfigured for performing a medical function (e.g., the conveyance ofelectrical stimulation energy and/or sensing of a physiologicalparameter) via the medical lead. The AIMD further comprises anelectrically conductive case containing the interior electroniccircuitry. The AIMD further comprises at least one electrical terminalconfigured for electrically coupling the electronic circuitryrespectively to the lead electrode(s). The AIMD further comprises aninductive element coupled in series between the at least one electricalterminal and the case. The inductive element is configured forhindering, and preferably preventing, the shunting of electrical currentfrom the terminal(s) to the case that has been induced byelectromagnetic interference (EMI) impinging on the medical lead.

In one embodiment, the inductive element is electrically coupled inseries between the interior electronic circuitry and one of theelectrical terminal(s) and the case, and the interior electroniccircuitry is configured to operate at a frequency below the frequency ofthe induced electrical current, thereby allowing electrical current tobe conveyed between the at least one electrical terminal and theinterior electronic circuitry. In one embodiment, the operatingfrequency of the interior electronic circuitry may be below 100 KHz, andthe frequency of the induced electrical current is above 1 MHz. Inanother embodiment, the operating frequency of the interior electroniccircuitry may be below 1 KHz, and the frequency of the inducedelectrical current is at least 64 MHz.

The AIMD further comprises at least one capacitive element electricallycoupled respectively in series between the electrical terminal(s) and acommon terminal, and another capacitive element electrically coupled inseries between the case and the common terminal, thereby electricallyisolating the electrical terminal(s) and case from each other at theoperating frequency. The AIMD may optionally comprise a connectorconfigured for removably connecting the lead to the at least oneelectrical terminal.

Other and further aspects and features of the invention will be evidentfrom reading the following detailed description of the preferredembodiments, which are intended to illustrate, not limit, the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate the design and utility of preferred embodimentsof the present invention, in which similar elements are referred to bycommon reference numerals. In order to better appreciate how theabove-recited and other advantages and objects of the present inventionsare obtained, a more particular description of the present inventionsbriefly described above will be rendered by reference to specificembodiments thereof, which are illustrated in the accompanying drawings.Understanding that these drawings depict only typical embodiments of theinvention and are not therefore to be considered limiting of its scope,the invention will be described and explained with additionalspecificity and detail through the use of the accompanying drawings inwhich:

FIG. 1 is a plan view of a Spinal Cord Stimulation (SCS) systemconstructed in accordance with one embodiment of the present inventions;

FIG. 2 is a plan view of the SCS system of FIG. 1 in use within apatient;

FIG. 3 is a plan view of an implantable pulse generator (IPG) and threepercutaneous stimulation leads used in the SCS system of FIG. 1;

FIG. 4 is a plan view of an implantable pulse generator (IPG) and asurgical paddle lead used in the SCS system of FIG. 2;

FIG. 5 is a block diagram of one filter arrangement that can be used inthe IPG to prevent shunting of electrical current to the IPG case; and

FIG. 6 is a block diagram of one filter arrangement that can be used inthe IPG to prevent shunting of electrical current to the IPG case.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The description that follows relates to a spinal cord stimulation (SCS)system. However, it is to be understood that the while the inventionlends itself well to applications in SCS, the invention, in its broadestaspects, may not be so limited. Rather, the invention may be used withany type of implantable electrical circuitry used to stimulate tissue.For example, the present invention may be used as part of a pacemaker, adefibrillator, a cochlear stimulator, a retinal stimulator, a stimulatorconfigured to produce coordinated limb movement, a cortical stimulator,a deep brain stimulator, peripheral nerve stimulator, microstimulator,or in any other neural stimulator configured to treat urinaryincontinence, sleep apnea, shoulder sublaxation, headache, etc.

Turning first to FIG. 1, an exemplary spinal cord stimulation (SCS)system 10 generally includes one or more (in this case, three)implantable stimulation leads 12, an active implantable medical device(AIMD), and in particular, a pulse generating device in the form of animplantable pulse generator (IPG) 14, an external control device in theform of a remote controller RC 16, a clinician's programmer (CP) 18, anexternal trial stimulator (ETS) 20, and an external charger 22.

The IPG 14 is physically connected via one or more lead extensions 24 tothe stimulation leads 12, which carry a plurality of electrodes 26arranged in an array. The stimulation leads 12 are illustrated aspercutaneous leads in FIG. 1, although as will be described in furtherdetail below, a surgical paddle lead can be used in place of thepercutaneous leads. As will also be described in further detail below,the IPG 14 includes pulse generation circuitry that delivers electricalstimulation energy in the form of a pulsed electrical waveform (i.e., atemporal series of electrical pulses) to the electrode array 26 inaccordance with a set of stimulation parameters.

The ETS 20 may also be physically connected via the percutaneous leadextensions 28 and external cable 30 to the stimulation leads 12. The ETS20, which has similar pulse generation circuitry as the IPG 14, alsodelivers electrical stimulation energy in the form of a pulse electricalwaveform to the electrode array 26 accordance with a set of stimulationparameters. The major difference between the ETS 20 and the IPG 14 isthat the ETS 20 is a non-implantable device that is used on a trialbasis after the stimulation leads 12 have been implanted and prior toimplantation of the IPG 14, to test the responsiveness of thestimulation that is to be provided. Thus, any functions described hereinwith respect to the IPG 14 can likewise be performed with respect to theETS 20.

The RC 16 may be used to telemetrically control the ETS 20 via abi-directional RF communications link 32. Once the IPG 14 andstimulation leads 12 are implanted, the RC 16 may be used totelemetrically control the IPG 14 via a bi-directional RF communicationslink 34. Such control allows the IPG 14 to be turned on or off and to beprogrammed with different stimulation parameter sets. The IPG 14 mayalso be operated to modify the programmed stimulation parameters toactively control the characteristics of the electrical stimulationenergy output by the IPG 14. As will be described in further detailbelow, the CP 18 provides clinician detailed stimulation parameters forprogramming the IPG 14 and ETS 20 in the operating room and in follow-upsessions.

The CP 18 may perform this function by indirectly communicating with theIPG 14 or ETS 20, through the RC 16, via an IR communications link 36.Alternatively, the CP 18 may directly communicate with the IPG 14 or ETS20 via an RF communications link (not shown). The clinician detailedstimulation parameters provided by the CP 18 are also used to programthe RC 16, so that the stimulation parameters can be subsequentlymodified by operation of the RC 16 in a stand-alone mode (i.e., withoutthe assistance of the CP 18).

For purposes of brevity, the details of the RC 16, CP 18, ETS 20, andexternal charger 22 will not be described herein. Details of exemplaryembodiments of these devices are disclosed in U.S. Pat. No. 6,895,280,which is expressly incorporated herein by reference.

As shown in FIG. 2, the stimulation leads 12 are implanted within thespinal column 42 of a patient 40. The preferred placement of thestimulation leads 12 is adjacent, i.e., resting near, the spinal cordarea to be stimulated. Due to the lack of space near the location wherethe stimulation leads 12 exit the spinal column 42, the IPG 14 isgenerally implanted in a surgically-made pocket either in the abdomen orabove the buttocks. The IPG 14 may, of course, also be implanted inother locations of the patient's body. The lead extensions 24 facilitatelocating the IPG 14 away from the exit point of the stimulation leads12. As there shown, the CP 18 communicates with the IPG 14 via the RC16.

Referring now to FIG. 3, the external features of the stimulation leads12 and the IPG 14 will be briefly described. Each of the stimulationleads 12 has eight electrodes 26 (respectively labeled E1-E8, E9-E16,and E17-E24). The actual number and shape of leads and electrodes will,of course, vary according to the intended application.

Further details describing the construction and method of manufacturingpercutaneous stimulation leads are disclosed in U.S. patent applicationSer. No. 11/689,918, entitled “Lead Assembly and Method of Making Same,”and U.S. patent application Ser. No. 11/565,547, entitled “CylindricalMulti-Contact Stimulation lead for Neural Stimulation and Method ofMaking Same,” the disclosures of which are expressly incorporated hereinby reference.

Alternatively, as illustrated in FIG. 4, the stimulation lead 12 takesthe form of a surgical paddle lead on which electrodes 26 are arrangedin a two-dimensional array in three columns (respectively labeled E1-E5,E6-E10, and E11-E15) along the axis of the stimulation lead 12. In theillustrated embodiment, five rows of electrodes 26 are provided,although any number of rows of electrodes can be used. Each row of theelectrodes 26 is arranged in a line transversely to the axis of the lead12. The actual number of leads and electrodes will, of course, varyaccording to the intended application. Further details regarding theconstruction and method of manufacture of surgical paddle leads aredisclosed in U.S. patent application Ser. No. 11/319,291, entitled“Stimulator Leads and Methods for Lead Fabrication,” the disclosure ofwhich is expressly incorporated herein by reference.

In each of the embodiments illustrated in FIGS. 3 and 4, the IPG 14comprises an outer case 44 for case the electronic and other components(described in further detail below). The outer case 44 is composed of anelectrically conductive, biocompatible material, such as titanium, andforms a hermetically sealed compartment wherein the internal electronicsare protected from the body tissue and fluids. In some cases, the outercase 44 may serve as an electrode. The IPG 14 further comprises aconnector 46 to which the proximal ends of the stimulation leads 12 matein a manner that electrically couples the electrodes 26 to the internalelectronics (described in further detail below) within the outer case44. To this end, the connector 46 includes one or more ports (threeports 48 or three percutaneous leads or one port for the surgical paddlelead) for receiving the proximal end(s) of the stimulation lead(s) 12.In the case where the lead extensions 24 are used, the port(s) 48 mayinstead receive the proximal ends of such lead extensions 24.

The IPG 14 further comprises interior electronic circuitry, such as amicrocontroller 52, a telemetry circuit 54, a battery 56, stimulationoutput circuitry 58, and other suitable components known to thoseskilled in the art. The microcontroller 52 executes a suitable programstored in memory (not shown), for directing and controlling theelectrical stimulation therapy performed by IPG 14. The telemetrycircuit 54 (including antenna is configured for receiving programmingdata (e.g., the operating program and/or neurostimulation parameters)from the RC 16 in an appropriate modulated carrier signal, anddemodulating the carrier signal to recover the programming data, whichprogramming data is then stored in memory. The battery 56, which may bea rechargeable lithium-ion or lithium-ion polymer battery, providesoperating power to IPG 14.

The stimulation output circuitry 58 that provides electrical stimulationenergy in the form of a pulsed electrical waveform to the electrodearray 26 in accordance with a set of stimulation parameters programmedinto the IPG 14. In the illustrated embodiment, the stimulation outputcircuitry 58 may either comprise independently controlled currentsources for providing stimulation pulses of a specified and knownamperage to or from the electrodes 26 or case 44, or independentlycontrolled voltage sources for providing stimulation pulses of aspecified and known voltage at the electrodes 26. The stimulation outputcircuitry 58 may further include charge recovery circuitry (not shown)to provide charge balancing of the electrodes and recovery of chargefrom the tissue. The interior electronic circuitry may include circuitrycapable of performing medical functions other than generating anddelivering electrical stimulation energy to tissue. For example, theinterior electronic circuitry may include monitoring circuitry (notshown) for measuring electrical parameter data (e.g., electrodeimpedance and/or electrode field potential).

The stimulation parameters in accordance with which the stimulationoutput circuitry 58 generates the pulsed electrical waveform to theelectrode array 26 may, e.g., comprise electrode combinations, whichdefine the electrodes that are activated as anodes (positive), cathodes(negative), and turned off (zero), percentage of stimulation energyassigned to each electrode of the array of electrodes 26 (fractionalizedelectrode configurations), and electrical pulse parameters, which definethe pulse amplitude (measured in milliamps or volts depending on whetherthe IPG 14 supplies constant current or constant voltage to the array ofelectrodes 26), pulse width (measured in microseconds), pulse rate(measured in pulses per second), and burst rate (measured as themodulation on duration X and modulation off duration Y).

Electrical stimulation will occur between two (or more) activatedelectrodes, one of which may be the IPG case 44. Simulation energy maybe transmitted to the tissue in a monopolar or multipolar (e.g.,bipolar, tripolar, etc.) fashion. Monopolar stimulation occurs when aselected one of the lead electrodes 26 is activated along with the case44 of the IPG 14, so that stimulation energy is transmitted between theselected electrode 26 and the case 44. Bipolar stimulation occurs whentwo of the lead electrodes 26 are activated as anode and cathode, sothat stimulation energy is transmitted between the selected electrodes26. For example, an electrode on one lead 12 may be activated as ananode at the same time that an electrode on the same lead or anotherlead 12 is activated as a cathode. Tripolar stimulation occurs whenthree of 15 the lead electrodes 26 are activated, two as anodes and theremaining one as a cathode, or two as cathodes and the remaining one asan anode. For example, two electrodes on one lead 12 may be activated asanodes at the same time that an electrode on another lead 12 isactivated as a cathode.

The stimulation energy may be delivered between electrodes as monophasicelectrical energy or multiphasic electrical energy. Monophasicelectrical energy includes a series of pulses that are either allpositive (anodic) or all negative (cathodic). Multiphasic electricalenergy includes a series of pulses that alternate between positive andnegative. For example, multiphasic electrical energy may include aseries of biphasic pulses, with each biphasic pulse including a cathodic(negative) stimulation pulse and an anodic (positive) recharge pulsethat is generated after the stimulation pulse to prevent direct currentcharge transfer through the tissue, thereby avoiding electrodedegradation and cell trauma. That is, charge is conveyed through theelectrode-tissue interface via current at an electrode during astimulation period (the length of the stimulation pulse), and thenpulled back off the electrode-tissue interface via an oppositelypolarized current at the same electrode during a recharge period (thelength of the recharge pulse).

Significant to the present inventions, electrical current induced byelectromagnetic interference (EMI) impinging on the stimulation leads 12is prevented, or at the least hindered, from being shunted to the outercase 44 of the IPG 14, while the pulsed electrical stimulation currentis allowed to be conveyed between the interior electronic circuitry ofthe IPG 14 and the stimulation leads 12 and outer case 44. In thismanner, any heating of the case 44 due to the induction of electricalcurrent on the lead electrodes 26 by the EMI is eliminated or reduced,thereby eliminating or reducing the heating of tissue surrounding theIPG that may otherwise occur.

In particular, and with reference to FIG. 5, the stimulation outputcircuitry 58 is electrically coupled to electrical terminals 60, whichwhen the stimulation leads 12 are mated with the connector 46, are inturn electrically coupled to the corresponding electrodes 26 carried bythe leads 12. The stimulation output circuitry 58 is also electricallycoupled to the case 44. As briefly discussed above, the stimulationoutput circuitry 58 may generate and convey pulsed electrical energybetween selected lead electrodes 26 in a bipolar or tripolar manner, ormay generate and convey the pulsed electrical energy between selectedlead electrodes 26 and the case 44 in a monopolar manner. Direct current(DC) blocking capacitors 62 (ECC1-ECcase) are provided between thestimulation output circuitry 58 and the electrical terminals 60 and case44 to minimize electrical charge build up in the tissue.

Significantly, the inductive elements 64 (L1-Ln) are coupled in series(from the perspective of the electrical current induced on the leadelectrodes 26 as an electrical source and the potential electrical pathcreated between the lead electrodes 26 and the case 44) between therespective electrical terminals 60 and the case 44. In the embodimentillustrated in FIG. 5, the inductive elements 64 are also electricallycoupled between the stimulation output circuitry 58 and the respectiveelectrical terminals 60. In an alternative embodiment illustrated inFIG. 6, an inductive element 64 (L_(case)) is electrically coupledbetween the stimulation output circuitry 58 and the case 44. Capacitiveelements 68 (C1-C_(case)) are coupled in series between the electricalterminals 60 and case 44 and a common (ground) terminal 66 of thestimulation output circuitry 58. These capacitive elements 68 areselected to have sufficiently low capacitive value to have a highimpedance at frequencies of the electrical stimulation energy generatedby the stimulation output circuitry 58, thereby electrically isolatingthe electrical terminals 60 and the case 44 from each other at theseoperating frequencies. In a particular embodiment, the impedance of thecapacitive elements 68 at the highest frequency of electricalstimulation energy is selected to be at least ten times higher than thetissue impedance at this frequency (e.g., 500 pf-2000 pf). Although thecapacitive elements 68 are shown in FIGS. 5 and 6 as being respectivelycoupled between the inductive elements 64 and the DC blocking capacitors62, the capacitive elements 68 may be coupled between the respective DCblocking capacitors 62 and the stimulation output circuitry 58.

Because the frequency of the electrical stimulation energy generated bythe stimulation output circuitry 58 is substantially lower than theexpected frequency of the electrical current induced on the electricalterminals 60 by EMI impinging on the stimulation leads 12, the shuntingof EMI-induced electrical current to the case 44 is hindered, while theconveyance of the electrical stimulation energy conveyed between thestimulation output circuitry 58 and the electrical terminals 60, oralternatively the electrical stimulation energy conveyed between thecase 44 and the stimulation output circuitry 58, is allowed. Forexample, it is expected that the frequency of the electrical stimulationenergy will be below 100 KHz, while it is expected that the frequency ofthe EMI-induced electrical current will be above 1 MHz, and in the caseof EMI generated by an MRI scanner, at least 64 MHz (64 MHz for a 1.5Tesla scanner and 128 MHz for a 3 Tesla scanner).

Thus, the inductance value of each of the inductive elements 64 has arelatively low impedance compared to the impedance presented by the leadand tissue at the operating frequencies of IPG 14 (i.e., the frequencyof the electrical stimulation energy generated by the stimulation outputcircuitry 58 or the frequency of the electrical energy sensed by themonitoring circuitry (not shown)), allowing signals to pass between theinternal electronic circuitry and the electrical terminals 58 and/orcase 44, while the inductance value has a relatively high impedancecompared to the RF output impedance of the lead at the IPG-connectedcontacts at the EMI frequencies, which is typically in the 30 to 150 ohmrange. In the case where different EMI frequencies must be taken intoaccount, each of the inductive elements 64 should maintain a highimpedance for this range of EMI frequencies.

Based on these frequency parameters, the inductance values of theinductive elements 64 can be appropriately selected. For example, in theembodiment illustrated in FIG. 5, the inductance value of each inductiveelement 64 may be equal to at least 0.5 μH, preferably equal to at least1.0 μH, and most preferably equal to at least 3.0 μH. In the case of a1.5 T MRI scanner that operates at 64 MHz, a 0.5 μH inductor will havean impedance of 200 ohms, a 1.0 μH inductor will have an impedance of400 ohms, and a 3.0 μH inductor will have an impedance of 1200 ohms. Inthe embodiment illustrated in FIG. 6, the inductance value of theinductive element 64 may be in the range of 50 nH to 200 nH. In the caseof a 1.5 T MRI scanner that operates at 64 MHz, an inductor in the rangeof 50 nH to 200 nH will have an impedance in the range of 20-80 ohms.Ultimately, to effectively preventing heating of the tissue that wouldotherwise occur by shunting the induced current to the case 44, theimpedance of each inductive element 64 should maintain a high impedancerelative to the stimulation leads 12 and the casing-to-tissue impedance.The effective lead impedance may be the parallel combination ofimpedances measured from the lead electrodes 26 and case 44 back intothe stimulation leads 12 at the EMI frequencies, as this is theeffective impedance when the lead electrodes 26 and case 44 areconnected in parallel through the inductive elements 64.

The inductive elements 64 preferably do not have any core materials thatare subject to magnetic saturation when exposed to strong staticmagnetic fields (e.g., the static magnetic fields emitted byconventional MRI scanners), such that inductive elements 64 maintains ahigh impedance at the EMI frequencies. Preferably, each inductiveelement 64 has a sufficiently high self-resonant frequency to maintainadequate impedance at the EMI frequencies.

Notably, the embodiments illustrated in FIGS. 5 and 6 may have certainadvantages relative to each other. In particular, with respect to theFIG. 5 embodiment, in addition to preventing or hindering the electricalcurrent induced on the lead electrodes 26 from being conveyed to thecase 44, the inductive elements 64 (L1-Ln) also reduces the common modeRF voltage levels with respect to the case 44, thereby preventing orhinders the induced electrical current from being conveyed to thestimulation output circuitry 58 in the form of injected power, while thepulsed electrical stimulation current is allowed to be conveyed betweenthe interior electronic circuitry of the IPG 14 and the stimulationleads 12 and outer case 44. This could be advantageous to keepelectrical current low to the case 44 if there is a high straycapacitance from the stimulation output circuitry 58 to the case 44.With respect to the FIG. 6 embodiment, the use of only the inductiveelement 64 (L_(case)) minimizes the component count relative to the FIG.5 embodiment, which uses an inductive element 64 for each lead electrode26.

Although particular embodiments of the present inventions have beenshown and described, it will be understood that it is not intended tolimit the present inventions to the preferred embodiments, and it willbe obvious to those skilled in the art that various changes andmodifications may be made without departing from the spirit and scope ofthe present inventions. Thus, the present inventions are intended tocover alternatives, modifications, and equivalents, which may beincluded within the spirit and scope of the present inventions asdefined by the claims.

What is claimed is:
 1. An active implantable medical device (AIMD) foruse with a medical lead carrying at least one lead electrode,comprising: interior electronic circuitry configured for performing amedical function via the medical lead; an electrically conductive casecontaining the interior electronic circuitry; at least one electricalterminal configured for electrically coupling the electronic circuitryrespectively to the least one lead electrode; and an inductive elementcoupled in series between the at least one electrical terminal and thecase, the inductive element being configured for hindering the shuntingof electrical current from the at least one electrical terminal to thecase that has been induced by electromagnetic interference (EMI)impinging on the medical lead.
 2. The AIMD of claim 1, wherein theinductive element is electrically coupled in series between the interiorelectronic circuitry and one of the at least one electrical terminal andthe case, and the interior electronic circuitry is configured to operateat a frequency below the frequency of the induced electrical current,thereby allowing electrical current to be conveyed between the at leastone electrical terminal and the interior electronic circuitry.
 3. TheAIMD of claim 2, wherein the one of the at least one electrical terminaland the case is the at least one electrical terminal.
 4. The AIMD ofclaim 3, wherein the inductive element has an inductance value equal toat least 0.5 μH.
 5. The AIMD of claim 3, wherein the inductive elementhas an inductance value equal to at least 1.0 μH.
 6. The AIMD of claim3, wherein the inductive element has an inductance value equal to atleast 3.0 μH.
 7. The AIMD of claim 2, wherein the one of the at leastone electrical terminal and the case is the case.
 8. The AIMD of claim7, wherein the inductive element has an inductance value in the range of50 nH to 200 nH.
 9. The AIMD of claim 2, wherein the operating frequencyof the interior electronic circuitry is below 100 KHz, and the frequencyof the induced electrical current is above 1 MHz.
 10. The AIMD of claim2, wherein the operating frequency of the interior electronic circuitryis below 1 KHz, and the frequency of the induced electrical current isat least 64 MHz.
 11. The AIMD of claim 2, further comprising at leastone capacitive element electrically coupled respectively in seriesbetween the at least one electrical terminal and a common terminal, andanother capacitive element electrically coupled in series between thecase and the common terminal, thereby electrically isolating the atleast one electrical terminal and case from each other at the operatingfrequency.
 12. The AIMD of claim 1, wherein the inductive element beingis configured for preventing the shunting of induced electrical currentfrom the at least one electrical terminal to the case.
 13. The AIMD ofclaim 1, wherein the interior electronic circuitry is configured forconveying electrical stimulation energy between the at least oneelectrical terminal and the case.
 14. The AIMD of claim 1, wherein theinterior electronic circuitry is configured for sensing a physiologicalparameter between the at least one electrical terminal and the case. 15.The AIMD of claim 1, further comprising a connector configured forremovably connecting the lead to the at least one electrical terminal.